1. Field of the Invention
This invention relates to a cooling device, and more particularly to a cooling device for cooling laser diode arrays that release large quantities of heat and to a surface emitting device, configured from laser diode arrays, which comprises that cooling device.
2. Description of the Related Art
One application for the laser diode array is seen in light sources for exciting high-output solid lasers. In general, in such a high-output solid laser, when a high average output at the kW level is sought, it is necessary (a) that continuous laser light having a high intensity of 100 W/cm2 or greater be generated in the laser diode array used as the excitation light source, and (b) that a large emitting surface area of several tens of cm2 be available. Given the constraints of realizing a large light emitting surface area, a cooling device is indispensable in order both to release heat generated at a density of 100 to 200 W/cm2 and to control the rise in temperature of the laser diode array.
In order for surface emitting light sources configured from such high-output laser diode arrays to be widely used in this field, it is very important that the cost per unit of light output be kept down.
FIG. 1A is a diagram of a surface emitting device 118 comprising a plurality of conventional one-dimensional laser diode arrays (hereinafter called laser bars), while FIG. 1B represents the configuration of the main parts of such a cooling device 119 for that surface emitting device.
As diagrammed in FIG. 1B, the heat generated by a laser bar 22 is conducted by a laser bar support plate 120, from there passes through an insulating plate 121 and a heat sink 122, and is discharged after heat exchange with a cooling liquid 123. For that reason, the thermal resistance between the laser bar 22 and the cooling liquid 123 will inevitably become comparatively high, so that the temperature of the laser bar 22 readily rises due to heat generation.
However, with a surface emitting device 118 that employs a cooling device 119 structured so that it cools the back surface of a two-dimensional laser diode array configured from a plurality of such laser bars 22, a configuration that makes almost the entire surface a light emitting surface is possible, as seen from the light emitting surface side, as diagrammed in FIG. 1A, wherefore, when an attempt is made to deploy an even larger plurality of these surface emitting devices to configure a surface emitting light source having a larger light emitting surface area, there are restrictions such as the degree of layout freedom being low and it being difficult to realize a surface emitting light source having few non-emitting parts.
FIG. 2 gives an exploded view representing the configuration of another type of a surface emitting device 124 comprising a plurality of conventional laser bars 22. As diagrammed in FIG. 2, each of the plurality of laser bars 22 is thermally connected to a separate cooling unit 125. The cooling liquid is made to flow into the cooling units 125 from a cooling liquid induction port (not visible in FIG. 2) that is connected to an intake opening 8 provided commonly for each of the cooling units 125, passes more or less directly below the positions where the laser bars 22 are deployed, and is then discharged from a cooling liquid discharge port 17 that is connected to an outlet opening 9 provided commonly for each of the cooling units 125. Hence the thermal resistance between the laser bars 22 and the cooling liquid can be made lower, and it becomes possible to hold down temperature rises in the laser bars 22 due to heat generation to a low level.
However, with a surface emitting device 124 wherein cooling devices are employed that have a structure that cools with cooling liquid immediately below the individual laser bars 22 in this manner, the non-emitting areas as seen from the light emitting surface side become great because of the deployment, on the outside of the stacked plurality of cooling devices, of an induction pipeline 14 for inducting the cooling liquid in from the intake opening and a discharge pipeline 16 for discharging the cooling liquid from the outlet opening, and the deployment of a tightening bolt 20 to prevent leakage of cooling liquid from between the cooling units 125 and 125. For that reason, when an attempt is made to deploy an even larger plurality of these surface emitting devices to configure a surface emitting light source having a larger light emitting surface area, there are restrictions such as the degree of layout freedom being low and it being difficult to realize a surface emitting light source having few non-emitting parts.
In FIG. 2, the intervals in the cooling device are represented as being wider than at the time of actual use, in the interest of making the configuration easy to understand, and the diagram shows neither the sealing material used to prevent the cooling liquid from leaking out from between the cooling units 125 and 125 nor the wiring connection board connecting to an electrode on the side of the laser bars not connected to the cooling devices.
FIG. 3 provides an exploded view of an example configuration of the cooling device diagrammed in FIG. 2. As diagrammed in FIG. 3, the cooling unit 125 is made up of a first thin plate 126, a second thin plate 127, a third thin plate 128, a fourth thin plate 129, and a fifth thin plate 130, stacked sequentially from the top.
A cooling liquid intake opening 8 and outlet opening 9 are formed in the first thin plate 126.
In the second thin plate 127, an outlet opening 8 is formed in a position corresponding to the outlet opening 9 in the first thin plate 126. A cooling liquid flow channel 131 is also formed therein, extending from a position corresponding to the intake opening 8 of the first thin plate 126 such that the width thereof widens as the front end surface 12 is approached. Microchannels 133 are also formed therein, near a front end surface 12, along that front end surface 12.
In the third thin plate 128, an intake opening 8 and an outlet opening 9 are formed at positions corresponding to the cooling liquid intake opening 8 and outlet opening 9 of the first thin plate 126. A slit 132 is also formed therein, near the front end surface 12, along that front end surface 12.
In the fourth thin plate 129, an intake opening 8 is formed at a position corresponding to the intake opening 8 of the third thin plate 128. A cooling liquid passage 131 is also formed so as to extend from a position corresponding to the outlet opening 9 of the third thin plate 126 towards the front end surface 12 while increasing the width thereof.
In the fifth thin plate 130, an intake opening 8 and an outlet opening 9 are formed at positions corresponding to the cooling liquid intake opening 8 and outlet opening 9 of the first thin plate 126.
The first to fifth thin plates 126 to 130 described above, respectively, are made of a highly thermally conductive material such as copper, for example, and are mutually stacked together. Cooling liquid inducted from the intake openings 8 passes through the microchannels 133 in the second thin plate and is discharged from the outlet openings 9. When this occurs, the laser bar (not shown) deployed along the front end surface 12 of the fifth thin plate 130 is cooled. In the microchannels 133, very fine flow channels, having a width of 20 xcexcm or so, are formed, by laser processing or the like, in order to prevent the heat exchanging efficiency from falling due to a cooling liquid boundary layer.
FIG. 4 is an exploded view of the configuration of a cooling device disclosed in Japanese Patent Application Laid-Open No. 209531/1998. This cooling device is a different type from that diagrammed in FIG. 3 which is for cooling one one-dimensional laser diode array.
As diagrammed in FIG. 4, the cooling device 134 is configured by three plate-form members 135, 136, and 137 which are made of a material such as copper which exhibits high thermal conductivity. On the lower surface of the plate-form member 135 on the upper side (that is, the surface facing the plate-form member 136 on the inside) are formed grooves like the pattern of grooves 33 formed on the upper surface of the plate-form member 137 on the lower side (that is, the surface facing the plate-form member 136 on the inside).
The grooves 33 described above, which are formed on the lower surface of the plate-form member 135 on the upper side and on the upper surface of the plate-form member 137 on the lower side configure cooling water channels 138. Ridges 35 that divide the grooves 33 from adjacent grooves 33 on the lower surface of the plate-form member 135 on the upper side are connected thermally and mechanically to the plate-form member 136 in the middle. In the plate-form member 136 in the middle, instead of the slit 132 diagrammed in FIG. 3, a plurality of very small through holes 139 that are mutually independent is formed, and bridges 140 between those very small through holes 139 and 139 contribute both to thermal conductivity and to the prevention of deformity in this middle plate-form member 136.
Because of the structure described above, with the plate-form member 135 on the upper side and the plate-form member 136 on the inside, the thermal connection between the three plate-form members 135, 136, and 137 is improved. Accordingly, by efficiently dispersing the heat generated by the laser bar 22 within the wide scope of this cooling device 134, not only is outstanding cooling performance achieved, but the mechanical strength of the cooling device 134 is also improved.
With the cooling device 134 described in the foregoing, the process steps for forming the plate-form members can be performed by chemical etching, wherewith many process steps can be done simultaneously, and many cooling devices can also be assembled simultaneously, so low-cost manufacturing is possible.
With the cooling device 134, as described in the foregoing, cooling devices exhibiting outstanding cooling performance can be realized at low cost. However, even in surface emitting devices using this cooling device 134, more non-emitting areas develop, as seen from the light emitting surface side, just as with the surface emitting device diagrammed in FIG. 2. That being so, when an attempt is made to deploy an even larger plurality of these surface emitting devices to configure a surface emitting light source having a larger light emitting surface area, because the degree of layout freedom is low, there are restrictions such as that it is difficult to realize a surface emitting light source having few non-emitting parts.
In the surface emitting device comprising the cooling device 119 diagrammed in FIG. 1B described earlier, on the other hand, it is possible to make the configuration one wherein almost the entire surface is a light-emitting surface, seen from the light-emitting surface side, as in the surface emitting device 118 diagrammed in FIG. 1A. That being so, when an attempt is made to deploy an even larger plurality of these surface emitting devices 118 to configure a surface emitting light source having a larger light emitting surface area, the degree of layout freedom is high, wherefore there is a benefit in being able to make a surface emitting light source having few non-emitting parts.
On the other hand, however, the heat generated by the laser bar 22 is conducted through the bar support plate 120, from there passes through the insulating plate 121 and heat sink plate 122, and is discharged after heat exchange with the cooling liquid 123, wherefore the thermal resistance between the laser bar 22 and the cooling liquid 123 inevitably becomes higher. As a result, there is a limitation on the electrical current that can be input into the laser bar 22, and it becomes very difficult to raise the light output per laser bar 22.
Thereupon, when the thickness of the bar support plate 120 is made thicker in an attempt to lower the thermal resistance, even if only slightly, in order to raise the light output per laser bar 22, the laser bar 22 pitch becomes greater, and, as a consequence, the light output intensity that is the light output per unit light emitting surface area in the surface emitting device is still kept low, so there is still a problem.
Furthermore, when the electric current input is increased and the light output is set high in order to avoid this problem even if only to a slight extent, problems arise in that the laser bar temperature becomes higher, the speed of laser bar property deterioration rises, and reliability declines.
In the surface emitting device comprising the cooling device 119 diagrammed in FIG. 1B, moreover, an assembly operation is necessary to densely implant the bar support plates 120 to which the laser bars 22 are bonded in the insulating plate by soldering or the like. This is not an efficient operation, so the assembly requires considerable time, which pushes costs up. This is a problem.
In the surface emitting device comprising the cooling device 119 diagrammed in FIG. 1B described earlier, furthermore, it is very difficult to preliminarily inspect and select the individual laser bars 22 in their actual working condition, and the operation of changing out some laser bars 22 whenever their characteristics deteriorate after they are assembled into the surface emitting device is also very difficult, causing surface emitting device yield to decline and costs to be higher. These are problems.
In the surface emitting device 124 comprising a cooling device wherein a plurality of the cooling units 125 diagrammed in FIG. 2 and described earlier, on the other hand, the laser bars 22 are thermally connected to the individual cooling units 125, respectively, wherefore the cooling liquid can be made to flow more or less directly below the positions where the laser bars 22 are deployed. Accordingly, if improvements are made so that the heat exchange between the cooling liquid and the inner walls of the water channels inside the cooling units near the laser bars 22 is done efficiently, it becomes possible to keep down the heat resistance between the laser bar 22 and the cooling liquid, thereby holding temperature rises in the laser bars 22 due to heat generation down to a low level.
That being so, as based on this surface emitting device 124, even in a condition wherein multiple laser bars 22 are deployed with a comparatively small pitch, the current input can be increased and the light output per laser bar 22 increased, without sacrificing the reliability of the laser bars 22, as a consequence whereof the advantage is gained of being able to realize a surface emitting device that exhibits high light output intensity. On the down side, however, as diagrammed in FIG. 2, the non-emitting areas as seen from the light emitting surface side become great because of the deployment, on the outside of the stacked plurality of cooling units 125, of a pipeline for inducting the cooling liquid in the intake opening 8 and a pipeline for discharging the cooling liquid from the outlet opening 9, and the deployment of a tightening bolt 20 to prevent leakage of cooling liquid from between the cooling units 125 and 125. As a consequence, when an attempt is made to deploy an even larger plurality of these surface emitting devices to configure a surface emitting light source having a larger light emitting surface area, the degree of layout freedom will be low and, depending on the shape of the surface emitting light source required, it will be difficult to realize a surface emitting light source having few non-emitting parts, which is a problem.
The size of the light-emitting surface in the direction perpendicular to the direction in which the cooling units are stacked can be made large by deploying a plurality of the surface emitting devices diagrammed in FIG. 2 adjacent to each other in that direction. When an attempt is made to make the size of the light-emitting surface larger in the direction in which the cooling units are stacked, however, a plurality of the surface emitting devices diagrammed in FIG. 2 cannot be deployed proximately in that direction, whereupon a wide non-light-emitting part develops, and a problem arises in that the solid laser medium can no longer be uniformly excited.
In this case, it is possible to increase the number of stacked cooling units, and to some extent make the size of the light-emitting surface in the direction in which the cooling units are stacked larger. However, the upper limit on the number of cooling units that can be stacked is normally about 30. When an attempt is made to stack a greater number than that, problems are encountered in that it becomes very difficult to evenly supply the cooling liquid to the cooling units, and assembly also becomes difficult.
Furthermore, when, in the surface emitting device 124 comprising the cooling device diagrammed in FIG. 2, cooling units are used which have the microchannels 133 so that the heat exchange between the cooling liquid and the inner walls of the water channels inside the cooling units near the laser bars can be done efficiently, as in the cooling unit 125 diagrammed in FIG. 3 described earlier, problems arise in that it is very difficult to form the very fine structures of the microchannels 133 by conventional chemical etching, whereupon laser processing is used, wherefore it is necessary to process the thin plates 126 to 130 one plate at a time, and processing costs escalate.
A minimum of five of the thin plates configuring the cooling unit 125 are necessary. However, when many thin plates are stacked together in this manner, problems arise in that the yield declines when bonding these together in an airtight structure so that the cooling liquid will not leak, and the cost of manufacturing the surface emitting device increases.
In more specific terms, because the thin plates are made of copper, they are readily deformed by the heat and pressure brought to bear on them during processing, particularly when joining them together by diffusion welding or the like. The parts corresponding to the adjacent copper thin plates that cover the cooling liquid flow channels cut out by laser beam are particularly susceptible to being crushed, and similar deformation readily occurs in the cooling unit front end surface 12 of the thin plate 128 wherein the slot 132 is formed. Furthermore, such deformations in the cooling unit 125 distort the laser bar 22 mounted thereon and, as a result, cause the laser bar characteristics to deteriorate.
When the cooling device 134 diagrammed in FIG. 4, noted earlier, is used instead of the cooling unit 125 diagrammed in FIG. 3, the thermal connection among the three plate-form members 135 to 137 described earlier is improved. This is due to the following two reasons.
(a) The ridges 35 forming the grooves 33 configuring the cooling water channels 138 on the plate-form member 137 on the lower side are connected thermally and mechanically to the plate-form member 136 on the inside that is deployed on the upper side of that plate-form member 137.
(b) In the plate-form member 136 on the inside, as noted above, multiple very small through holes 139 which are mutually independent are provided instead of the slit, and the bridges 140 between those very small through holes 139 contribute both to thermal conductivity and to the prevention of deformity in the plate-form member 136.
With the cooling device 134 diagrammed in FIG. 4, moreover, by efficiently diffusing the heat generated by the laser bar 22 within the wide scope of the cooling unit 134, outstanding cooling performance is realized, even without using the microchannels 133, and the mechanical strength of the cooling unit 134 is enhanced.
With the cooling unit 134 described above, furthermore, the process steps for forming the plate-form members described above can by done by chemical etching wherewith many such processes can be done simultaneously, and many cooling units can also be assembled simultaneously, wherefore low-cost manufacturing is possible.
Thus, with the cooling unit 134 described in the foregoing, a cooling unit can be realized that exhibits outstanding cooling performance at low cost, whereupon almost all of the problems associated with the cooling unit 125 diagrammed in FIG. 3 are resolved. Nevertheless, even in this surface emitting device in which this cooling unit 134 is used, the non-light-emitting area as seen from the light-emitting surface side increases in a similar manner to FIG. 2, and, when an attempt is made to deploy an even larger plurality of these surface emitting devices to configure a surface emitting light source having a larger light emitting surface area, there are restrictions, due to the fact that the degree of layout freedom is low, such as that it will in some cases be difficult to realize a surface emitting light source having few non-emitting parts.
An object of the present invention is to provide a cooling device that has high cooling capacity and also facilitates a large degree of freedom in the layout of the objects to be cooled, and is also to provide together with a surface emitting device made up of a plurality of laser diode arrays comprising that cooling device.
A more specific object thereof, in cases where the objects to be cooled are laser diode arrays, is to realize, at low cost, a large-scale surface emitting light source that exhibits high light output intensity per laser diode array as a consequence of there being few non-light-emitting parts as seen from the light-emitting surface side, and of the thermal resistance between the laser diode arrays and the cooling liquid being low.
In order to achieve the objects stated above, the cooling device according to the present invention is a cooling device that is formed by stacking up cooling units to which objects to be cooled are mounted, comprising: a first cooling liquid supply passage deployed so as to pass through first portions of the cooling units in order to conduct the cooling liquid about the periphery of each of the objects to be cooled; a cooling liquid discharge passage deployed so as to pass through second portions of the cooling units in order to discharge the cooling liquid that has cooled each of the objects to be cooled to the outside of the cooling device; and a second cooling liquid supply passage that passes through third portions of the cooling units, and is connected to the first cooling liquid supply passage, wherewith cooling liquid is supplied from the outside.
The cooling device according to the present invention is configured as described above, wherefore the degree of freedom in the layout of the objects to be cooled can be increased while maintaining the cooling capacity relative to the objects to be cooled. Here, each of the cooling units noted above can be made to be such that it comprises a primary cooling unit that has two holes provided therein that open respectively to the first cooling liquid supply passage and the cooling liquid discharge passage, and a secondary cooling unit that has at least three holes provided therein that open, respectively, to the first cooling liquid supply passage, the cooling liquid discharge passage, and the second cooling liquid supply passage.
The secondary cooling units, furthermore, can be made such that they connect the first cooling liquid supply passage and the second cooling liquid supply passage, and also comprise a plurality of flow channels divided by ridges. Thus, by forming a plurality of flow channels divided by ridges, cooling units deployed adjacently to each other are supported by the ridges, wherefore the deformation thereof can be prevented.
The cooling device described in the foregoing can also be made such as to comprise pressure bonding means for pressure bonding the stacked cooling units.
By such means, cooling liquid can be prevented from leaking from between the cooling units. A belt that securely tightens the stacked cooling units can be made the pressure bonding means noted above.
If the cooling units are securely tightened together by a belt or belts as described above, not only can the cooling unit structure be simplified, but the widths of the first and second cooling liquid supply passages and cooling liquid discharge passage described earlier can be made wider, so that pressure loss can be reduced.
The cooling device described in the foregoing can also be made such that, in addition to a plurality of cooling units being deployed so that they are lined up, a cooling liquid induction pipeline is also provided which is connected to all of the second cooling liquid supply passages contained in the cooling units that are deployed so that they are lined up.
The cooling device described in the foregoing can also be made such that a second cooling liquid discharge passage is provided that discharges the cooling liquid to the outside of the cooling device, being connected to the cooling liquid discharge passage and also passing through fourth portions of the cooling units.
The cooling device described in the foregoing can also be made such that the cooling units comprise a first cooling unit wherein are provided two holes that open, respectively, to the first cooling liquid supply passage and the cooling liquid discharge passage, a second cooling unit wherein are provided at least three holes that open, respectively, to the first cooling liquid supply passage, the cooling liquid discharge passage, and the second cooling liquid supply passage, and a third cooling unit wherein are provided at least three holes that open, respectively, to the first cooling liquid supply passage, the second cooling liquid supply passage, and the second cooling liquid discharge passage that is connected to the cooling liquid discharge passage described earlier.
The cooling device described in the foregoing can also be made such that, in addition to a plurality of cooling units being deployed so that they are lined up, a cooling liquid discharge pipeline is also provided which is connected to all of the second cooling liquid discharge passages contained in the cooling units that are deployed so that they are lined up.
If the configuration is made in this way, the transfer of cooling liquid between the cooling device and the outside can be effected easily.
In order to achieve the objects stated above, furthermore, the surface emitting device according to the present invention is a surface emitting device wherein laser diode arrays mounted to cooling units are stacked up, comprising: a first cooling liquid supply passage deployed so as to pass through first portions of the cooling units in order to conduct the cooling liquid about the periphery of each of the laser diode arrays; a cooling liquid discharge passage deployed so as to pass through second portions of the cooling units in order to discharge the cooling liquid that has cooled each of the laser diode arrays to the outside of the surface emitting device; and a second cooling liquid supply passage that passes through third portions of the cooling units, and is connected to the first cooling liquid supply passage, wherewith cooling liquid is supplied from the outside.
The surface emitting device according to the present invention is configured as described in the foregoing, wherefore a surface emitting device can be obtained wherein laser diode arrays are deployed in higher density.
Here, the surface emitting device described above can be made to comprise a belt or belts for securely tightening the stacked cooling units.
Also, the surface emitting device described above can be made to comprise also a lens or lenses held by the belt or belts noted above, deployed facing the surface formed by the stacked laser diode arrays.
With the configuration described above, a high-output surface emitting device can be obtained which generates the desired light.
Based on the present invention, cooling devices can be realized which have high cooling capacity and allow a great degree of freedom in deploying the objects to be cooled. When those objects to be cooled are laser diode arrays, a surface emitting device can be obtained which has high light output intensity and little non-light-emitting area as viewed from the light-emitting surface formed by the laser diode arrays.
As based on the present invention, moreover, while effecting a structure wherein there are few non-light-emitting parts as viewed from the light-emitting surface side, as noted above, cooling liquid can be supplied from the back side so that cooling liquid can be supplied as far as immediately below all of the laser diode arrays. Accordingly, a surface emitting device can be obtained which exhibits uniform light emission intensity, wherewith the occurrence of light emission intensity irregularity induced by laser diode array temperature differences caused by differences in cooling capacity can be suppressed.
Also, if the flow channels formed in the second cooling unit become too wide, the mechanical strength of the cooling unit will decline, so that deformations will arise, such as the cavity portion inside the flow channels being crushed during the process of joining the cooling units together wherein pressure is applied and the temperature is raised, for example. That will lead to the flow channels becoming blocked or to cooling liquid leakage, and the assembly yield will decline. That being so, the multiple flow channels are divided by the ridges to prevent cooling unit deformation, and it thereby becomes possible to fabricate highly reliable cooling units having good mechanical strength with good production yield.
Also, by configuring the cooling units of electrically conductive materials having a coefficient of thermal conductivity of 1.5 W/cm/K or greater, it becomes possible to efficiently discharge the heat generated by the objects to be cooled into the cooling liquid, and outstanding cooling performance can be realized. All of the cooling units stacked up can be fabricated from the same material, moreover, making it possible to unify the fabrication processes and to reduce fabrication costs.
Also, by additionally providing pressure bonding means for pressure bonding the stacked cooling units, the cooling liquid can be inducted as far as immediately below the objects to be cooled, and a cooling device that exhibits very high cooling performance can be obtained, even without using a tightening bolt to prevent cooling liquid leakage. That being so, by providing such a cooling device as this, in cases where the objects to be cooled are laser diode arrays, surface emitting devices can be obtained which exhibit high light output intensity, having little non-light-emitting area as seen from the light-emitting surface side.
By using a belt or belts that securely tightens the cooling units as the pressure bonding means, it becomes possible to tighten the cooling units across a broad width, whereupon, not only is the problem of cooling unit warping eliminated, but, when such cooling units are used, surface emitting devices can be obtained wherein the non-light-emitting surface area is even smaller as seen from the light-emitting surface. It is also then unnecessary to provide holes in the cooling units for a tightening bolt, the overall length of the cooling units can be made short, and cooling unit fabrication costs can be reduced.
Also, the belt or belts noted above can also be used as an outer protective container for protecting the laser diode arrays. While reducing the danger of inadvertently damaging the laser diode arrays during maintenance, etc., a surface emitting device can be realized which, inclusive of the outer protective container, has few non-light-emitting parts as seen from the light-emitting surface. And, as compared to loading the surface emitting device inside a separately prepared outer protective container, surface emitting devices equipped with outer protective containers can be obtained at lower cost.
Also, by using the belt described in the foregoing that serves also as an outer protective container to hold a lens or lenses for collimating the laser beams from the laser diode arrays and converting them to parallel light, etc., the lens or lenses can be deployed with good precision, without making the overall device size large, and low-cost surface emitting devices can be obtained.
Also, by having a plurality of cooling units deployed so that they are lined up, and further providing a cooling liquid induction pipeline that is connected to all of the second cooling liquid supply passages, a cooling device is obtained wherewith the cooling liquid can be inducted from the outside without increasing the number of pipelines. By so doing, many of the cooling devices described above can be arranged and devices wherein the objects to be cooled are highly integrated can be easily fabricated.